1. Field
The present description is related to the field of video processing and in particular to upsampling chroma signals to produce higher resolution or progressive frames in video signals.
2. Related Art
Video signals may be composed of luma (Y) and two chroma (Cb, Cr) signals. The luma signal represents illumination or brightness and the chroma signals represent color aspects of the signals. These signals may be in any of a variety of different sampling formats. 4:4:4 YCbCr is used to designate a format in which the three video components, Y, Cb, Cr, have the same sampling rate. Since human perception of brightness (the luma aspect) is more sensitive than perception of colors (the chroma aspect), the chroma portion of the signal is often subsampled to reduce the amount of bandwidth needed to process the signal and to store it. 4:2:2 is a popular format used in many professional video cameras and storage devices and indicates that the two chroma components are sampled at half the rate, horizontally, of the luma component. 4:1:1 subsamples chroma information at one quarter the rate, horizontally, of the luma information and is commonly used in professional portable video news cameras.
4:2:0 YCbCr is commonly used in broadcast video, DVDs (Digital Video Disks) and other popular entertainment devices. It is supported by most MPEG (Motion Picture Experts Group) compression formats as well as many other formats. In 4:2:0 video, the Cb and Cr components are both subsampled at half the rate of the luma, both vertically and horizontally.
Chroma upsampling may be used in the field of video processing to convert video from, for example, 4:2:0 to 4:2:2, or higher resolution. This may be done to accommodate a particular type of display, to allow different video sources to be combined into a single program, or to improve the quality of the final displayed video imagery. Since 4:2:0 video has one half of the vertical chroma resolution of 4:2:2 video, spatial vertical filtering is typically used to fill in the additional chroma pixels. If the original signal is interlaced, then deinterlacing might be used to get the final progressive frames.
Chroma upsampling becomes difficult in the context of deinterlacers which interpolate video from an interlaced format e.g. 480i or 1080i to a progressive format e.g. 480p or 1080p. Conventional chroma upsampling methods apply simple linear filtering techniques to interpolate missing pixels in a single video frame from reference pixels in the spatial neighborhood in the same frame. In other words, the value of a chroma pixel is derived from the values of the pixels that are next to it or nearby. Some advanced chroma upsampling techniques consider local content information to adapt the weighting used for local filtering to improve the picture quality. However, these spatial techniques that rely on the spatial neighborhood ignore temporal information which is often a significant aspect of video signals.